The goal of this R21 proposal is to study the effects of anthrax proteins on the adult and fetal emerging immune system. The long-term goal is to understand the effects of B. anthracis infection on the adult and fetal emerging immune systems in vivo, and to clarify the mechanisms that could provide hurdles on the emerging immune system during vaccination against B. anthracis. The over-arching hypothesis states that lethal toxins (LT) of anthrax affect the emerging immune system of adult and fetus. In the adult, alterations could occur at the level of the immature and mature hematopoietic stem cell (HSC); mesenchymal stem cells (MSC), which appear to be the cells that limit immune responses in the BM; and BM stromal cells (supporting cells of hematopoiesis). Fetal immune system is included because anthrax infection could occur in a pregnant individual. In fetus, LT could affect hematopoiesis by interrupting the differentiation of hemangioblasts to HSC. LT could affect both the adult and fetal hematopoietic system directly through anthrax receptor (ATR) and/or through molecules that mimic ATR (based on bioinformatics studies). The following specific aims will be investigated: Aim 1 will use phenotyping studies to identify subsets of BM cells that bind PA through molecular mimicry of ATR or directly to the ATR receptor. Aim 2 will perform functional studies to identify areas of the adult hematopoietic system that are affected by anthrax proteins. Aim 3 will address dysfunctions within the fetal developing system by anthrax proteins. These studies will be addressed with hemangioblasts, which will be derived from federal-approved embryonic stem cells. Together the proposed aims will formulate focused information to device an R01 application to perform mechanistic studies to understand the mechanisms by which anthrax proteins can affect hematopoiesis. This study will also form the basis to develop in vivo model with human hematopoietic system to study anthrax infection and also to improve responses to anthrax vaccines.